Among aerobic animals, the lung functions to provide an interface for the exchange of gases between blood and the atmosphere. The agents of this exchange are numerous small sacs termed alveoli (in adult humans about 150,000,000 per lung) that provide a gas permeable-liquid impermeable barrier between the gas and liquid phases. Between the alveoli are numerous capillaries carrying deoxygenated blood to the lung from the tissues and oxygenated blood from the alveoli to the tissues. The partial pressure of oxygen in the lungs is approximately 100 mm Hg at sea level; at this pressure the binding of oxygen by hemoglobin in the erythrocytes is favored. The alveoli thus provide a means for presenting the oxygen to hemoglobin to permit the conversion of deoxyhemoglobin to hemoglobin. Because the exchange occurs at the surface of the gas/blood barrier, alveoli have evolved as a means for providing extremely high surface area in a compact overall area, thus maximizing possible gas exchange. Lack of adequate gas exchange leads to disability which can progress to death.
Diseases that result in fewer alveoli and/or the deflation of alveoli therefore are quite serious, and are common causes of inadequate oxygenation and frequently resultant in disability and death. Among such diseases are chronic lung diseases such as bronchopulmonary dysplasia (BPD), asthma, emphysema, and idiopathic pulmonary fibrosis. BPD is a disease of prematurely born infants, and is characterized mainly by a failure of the infant to form a sufficient number of appropriately-sized alveoli. Emphysema, a disease of middle and advanced age, appears to be due to progressive proteinase-induced alveolar destruction. Many of these pathological conditions are characterized by the dedifferentiation of lipofibroblasts to myofibroblasts, resulting in various fibroses.
A marker for the onset or progression of these diseases, and/or for the response of a patient (or veterinary subject) to various therapeutic regimes would greatly facilitate the treatment of such conditions.